Graphene, a two-dimensional sp2 hybridized lattice of carbon atoms, has generated intense interest due to its unique electronic, mechanical, chemical, and catalytic properties. Recent synthetic efforts have focused on the development of high-yield and scalable methods of generating graphene. These techniques include the direct exfoliation of either chemically modified or pristine graphene directly into various solvents. For example, graphene oxide (GO) can be exfoliated from graphite via acidic treatments. The resulting GO flakes contain hydroxyl, epoxyl, carbonyl, and carboxyl groups along the basal plane and edges that render GO strongly hydrophilic. The ease of dispersing GO in solution has facilitated the preparation of GO thin films and GO-polymer nanocomposites with interesting and potentially useful mechanical properties. However, due to the defects and consequent disruption of the graphene band structure introduced during oxidation, GO is a poor electrical conductor. Although the level of oxygenation can be partially reversed through additional chemical reduction steps, significant quantities of structural and chemical defects remain. Moreover, the electrical conductivity of reduced GO flakes is less than optimal and is certainly deficient by comparison to pristine graphene.
In an effort to circumvent such GO limitations, recent efforts have focused on direct solution-phase exfoliation of pristine graphene. Graphene sheets can be extracted using superacids, by sonication in surfactant solutions and through use of organic solvents. For example, superacids have demonstrated an unprecedented graphene solubility of 2 mg/mL through the protonation and debundling of graphitic sheets. However, the resulting solutions are not ideally suited for additional processing due to their acidity-dependent solubility and high reactivity. Direct exfoliation of graphene in surfactant solutions and select organic solvents has also been demonstrated with concentrations up to 0.3 mg/mL and 1.2 mg/mL, respectively, but such concentrations are achieved only following prolonged sonication times—approaching 3 weeks in duration—or extended ultracentrifugation.
Processing complexities represent a bottleneck for fundamental studies and end-use applications that require well-dispersed, highly concentrated, pristine graphene solutions. Accordingly, there remains an on-going search in the art for an improved approach to graphene solution concentrations sufficient to better realize the benefits and advantages available from graphene and related material compositions.